As these two words have the same pronunciation it’s not uncommon to get confused with them, let's see their meanings and usage examples.
It's an informal way to refer to an advertisement, which means "A public promotion of some product or service".
- The ad appeared in the Super Bowl last night.
- To reach more people we should place an ad in the newspaper.
It's a verb, defined as "To join or combine (numbers) through addition."
- We can't forget to buy it today so please add it to the list.
- If you add too much sugar it won't taste good.